Basic slot type naming

From Erights

The type of slot produced by a "var" pattern is called SimpleSlot, which is a name left over from when such slots were the unmarked case in the language. They should be renamed, since there is nothing especially simple about them.

We should also reevaluate the naming of the other slot types, especially if guard-based auditing becomes official, introducing the slot type tentatively named CoercedSlot.